The invention relates to an electro-optical device comprising:
a light-transmitting substrate;
a stack of layers on the substrate, which stack comprises a first, light-transmitting electrode layer near the substrate, a second electrode layer and an organic electroluminescent layer between the first and the second electrode layer;
a hollow cover placed over the stack, which cover is connected to the substrate in a gastight manner and has a first wall extending along the stack; and
a water-binding getter accommodated in the cover so as to abut against the first wall.
Such an electro-optical device is disclosed in U.S. Pat. No. 5,882,761.
By injecting electrons through one electrode layer into the luminescent layer and injecting holes through the other electrode layer, light is generated in said layer by combining holes and electrons. A higher voltage across the electrodes results in a stronger light emission. This effect can be annihilated, however, by moisture resulting from the manufacturing process or moisture originating from the surroundings.
For this reason, in the known device, the stack of layers is situated in a gastight space formed by the substrate and the cover, and use is made of a water-binding getter.
The device is operated by means of control electronics, for example on a printed circuit board, enabling, for example, the voltage across the electrodes to be controlled or varied.
A drawback of the known device resides in that the combination of the device and the control electronics thus takes up too much space.
It is an object of the invention to provide an electro-optical device of the type described in the opening paragraph, which is designed so as to enable the amount of space taken up by the device and the associated control electronics to be limited.
In accordance with the invention, this object is achieved in that the first wall of the cover is provided with an indentation, wherein control electronics is accommodated so as to be electrically connected to the first electrode and the second electrode.
The design of the device enables a compact construction to be achieved. Besides, the device has the advantage that it yields a module wherein control electronics is present near the device""s electro-optical element to be operated. The module is a finished building block which can be fed to the assembly line of a larger device, such as a telephone.
The device may comprise a continuous first electrode layer and a continuous second electrode layer. This enables the device to be used as a light source, for example as a backlight. Alternatively, the first and/or the second electrode layer may be segmented, as a result of which images or characters can be formed by energizing segments. This enables the device to be used as a display. It is also possible for the segmentation to comprise essentially parallel lines in one layer and essentially parallel lines, extending transversely thereto, in the other layer, as a result of which characters and images are built up of points. The device may be monochrome or display colors, for example red, green and blue and mixed colors thereof, such as white and orange.
The first electrode layer may be a hole-injecting electrode layer, for example of a metal, such as gold, platinum or silver, which metals are light-transmitting at a small layer thickness, but preferably use is made of, for example, indium tin oxide because of the high light transmission of this material. Alternatively, organic electroconductive polymers may be used, such as polyaniline and poly-3,4-ethylenedioxythiophene (PEDOT) and polymers derived therefrom. The first electrode layer may be a double layer of, for example, indium tin oxide and PEDOT. Said double layer generally leads to an improved hole injection.
The second electrode layer may be an electron-injecting electrode layer, for example of a metal or a metal alloy having a low work function, such as Yb, Ca, Mg:Ag, Li:Al, Ba, or the layer is a laminate of various layers, such as Ba/Al or Ba/Ag.
The electroluminescent layer may basically comprise an organic electroluminescent material of either comparatively high or comparatively low molecular weight. Such a material may have an essentially conjugated skeleton, such as polythiophenes, polyphenylenes, polythiophenevinylenes, in particular, for example, blue-emitting poly(alkyl)fluorenes and/or red, green or yellow-emitting poly-p-phenylenevinylenes.
The substrate may be of, for example, a synthetic resin such as thermoplast, or of glass, such as quartz glass, or of a ceramic material.
For the getter use can be made of a hygroscopic substance which is fixed in the cover, for example, by compression or sintering. Examples of such a substance include BaO, CaO, CaSO4, Ba(ClO4) and Ca(ClO4).
A drawback of a glass cover, as used in the known device, is its comparatively large wall thickness, which is necessary for sufficient mechanical strength. This wall thickness also results in a comparatively large weight.
Therefore, it is favorable to manufacture the cover from a metal, for example, aluminum or chrome nickel steel. Chrome nickel steel is attractive because, in combination with a glass substrate, its thermal expansion is close to that of the substrate, so that the cover-substrate connection is capable of withstanding comparatively large temperature variations. Besides, in this case the cover can be readily manufactured from sheet material or strip material, for example by deep drawing.
It is favorable for the control electronics to be present on a flexible foil with conductor tracks connecting the control electronics with the first and the second electrode. This is attractive because the foil can be readily folded in two so as to cover the conductor tracks and thus separate them from the cover. As a result, no additional insulator has to be provided.
The control electronics, generally an I.C., can be fixed in the indentation by means of an adhesive. This measure has the advantage that it yields a robust, easily handled unit.
If the cover is made of a metal, the cover can be connected to the substrate by means of, for example, a sealing means wherein electrically insulating power is dispersed. Said powder may consist of, for example, glass beads. This enables the adhesive, for example an epoxy glue, to act directly on the cover and the substrate, without the need of special measures to preclude that the metal cover short-circuits conductors situated on the substrate and running to the electrodes. In this case, the powder acts as a spacer.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.